This application relates to a turbofan gas turbine engine. More particularly, the application relates to a reflex nozzle associated with a core nacelle of the engine.
Turbofan engines include a core nacelle at least partially surrounded by a fan nacelle. The core nacelle houses a compressor section and a turbine section that rotationally drives a turbofan. In high bypass turbofan arrangements, the turbofan creates most of the thrust provided by the turbofan engine.
Turbofan engines having a high bypass ratio with low fan pressure ratio engine cycles can have problems under descent idle conditions. Freestream and fan flow through the bypass flow path (between the core and fan nacelles) can choke the flow exiting the core nozzle under descent idle conditions. Further, the bypass flow can recirculate into the exit of the core nozzle. As a result, engine stability is adversely affected and a stall condition of the core can result.
The core nozzle includes an exterior surface that has a generally conical contour. To address the problem of descent idle conditions, some turbofan engines have used a small turnback at a terminal end of the conical nozzle. The turnback is provided by a relatively small exterior radius that is sloped to a lesser degree than the conical contour upstream from the turnback. The turnback has provided a negligible improvement and the core flow can still become choked during descent idle conditions.
What is needed is a turbofan engine that is not susceptible to the core flow becoming choked under descent idle conditions or other conditions in which the high bypass flow tends to suppress the flow through the core.